docs: def open(self, mode=None):
          """
          Open the container file.
  
          Args:
              mode (str): Either 'r' for read-only, 'w' for truncate and write or
                          'a' for append. (default: 'a').
                          If ``None``, uses ``self.mode``.
          """
  
          if mode is None:
              mode = self.mode
          elif mode not in ['r', 'w', 'a']:
              raise ValueError('Invalid mode! Modes: [\'a\', \'r\', \'w\']')
  
          if self._file is None:
              self._file = h5py.File(self.path, mode=mode)